1. Field of the Invention
The present invention relates to a radiation imaging apparatus for performing a so-called offset, the driving method thereof, and a radiation imaging system.
2. Description of the Related Art
In recent years, in general, there has been known a radiation imaging apparatus, which comprises a flat-type area sensor arranged with pixels in a matrix, which are composed of a conversion element and a TFT based on amorphous silicon and polysilicon on an insulating substructure such as a glass substrate. In this radiation imaging apparatus, radiation such as X-rays is converted into an electric charge, and the converted electric charge is subjected to matrix-driving by using the TFT serving as a switch element controlled by a driving control unit, so that the electric signal based on the electric charge is read, and is read out to a circuit unit.
Using such an apparatus, an object image can be obtained by the electric signal output from a read out circuit unit, but this image (electric signal output from the read out circuit unit) includes an offset component generated by the area sensor and the read out circuit unit. Since the image radiographed by actually irradiating it with the radiation includes the above-described offset component, it is necessary to perform an offset correction in order to remove the offset component from the radiographed image.
Heretofore, this offset correction has been performed by what can be broadly divided into two methods. A first offset correction method is a method of obtaining an image data for offset in advance. In this method, the electric signals based on the electric charges accumulated in the pixel in a state in which the radiation, or light based on the radiation, is not incident on the area sensor are read out by using the driving circuit unit and the read out circuit unit from the area sensor, thereby to obtain an image data FO for offset correction. The image data FO for offset correction thus obtained is stored in an image memory for offset. When the image data for offset correction is obtained in advance in this manner, it is often the case that a ROM is used for the image memory for offset.
After that, every time the radiation including the object image information or the light based on that radiation is incident, a read action is performed for the area sensor, the driving circuit unit, and the read out circuit unit. At this time, for every radiographing, a radiation image data Xn is stored in a radiation image memory. In an arithmetic operation unit, an arithmetic operation processing such as subtracting the image data FO for offset correction from the radiation image data Xn is performed, and the image data subjected to the arithmetic operation processing is displayed in a display unit such as a monitor.
In the above-described conventional example, since the image data FO for offset correction is obtained in advance, it is not necessary to obtain the image data for offset correction for each radiographing, and this is advantageous in performing a prompt radiographing. However, it is generally known that the offset component in the flat panel-type area sensor often changes due to factors such as a variance in time, variance in temperature, image lag (effect due to optical hysteresis of the preceding frame), and variance in defective pixels.
When a change occurs in the offset component of the above-described area sensor, the correction method based on the image data for offset correction stored in advance is not sufficient in terms of image quality, but often causes malfunctions. That is, in this case, the offset correction had often the opposite effect of lowering the image quality of the radiation image data.
Hence, in a second offset correction method, every time the irradiation with radiation or with light based on that radiation is performed, in other words, every time the radiation image is radiographed, both the radiation image data Xn and an image data Fn for offset correction are obtained so as to perform the offset correction. In this method, the radiation or the light based on that radiation is directed at the area sensor, and after that, the read out of a radiation image data X1 is performed, and this radiation image data X1 is stored in the radiation image memory. Subsequently, in a state in which the radiation or the light based on that radiation is not incident on the area sensor, an image data F1 for offset correction is obtained, and this image data F1 for offset correction is stored in the image memory for offset. When the content stored in the image memory for offset is rewritten every time in this manner, RAM is used for the memory.
After that, every time radiographing is performed in the operation unit, the arithmetic operation processing such as subtracting the image data Fn for offset correction from the radiation image data Xn is performed, and the image data subjected to the arithmetic operation processing is displayed in the display unit such as the monitor. For example, Japanese Patent Application Laid-Open No. 2002-301053 discloses a radiation imaging apparatus, which performs an offset correction by using the image data for offset correction obtained at the same time intervals as the output of the radiation image data during a period in which the radiation or the light based on that radiation is not irradiated.
In this second offset correction method, it is possible to prevent fluctuation of the image data for offset correction by the changes such as the above described variance in time, variance in temperature, image lag, and variance in defective pixels and avoid the lowering of the image quality of the radiation image data. However, on the other hand, in the second offset correction method, since it takes the same time as required in obtaining the radiation image data to obtain the image data for offset correction, it has been difficult to perform a prompt radiographing.